After a secondary battery is manufactured, a test is conducted to check whether or not foreign matter is mixed in the secondary battery. If any foreign matter is mixed, a short circuit may occur within the secondary battery.
A test method includes charging the secondary battery to a full charge state and then leaving the secondary battery standing. The voltage of the secondary battery is measured before and after it is left standing, and it can be determined on the basis of the difference between those voltages (voltage drop) whether or not foreign matter is mixed in the secondary battery. The voltage drop when the short circuit occurs due to the foreign matter is larger than the voltage drop in the secondary battery which includes no foreign matter.